Goodwill and Other Intangible Assets - Summary of Changes in Quanta's Goodwill (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2017	Sep. 30, 2017
Goodwill [Roll Forward]
Goodwill gross, beginning balance	$ 1,642,902
Accumulated impairment, beginning balance	(39,733)
Goodwill, beginning balance	1,603,169
Goodwill acquired	311,403	$ 311,400
Purchase price allocation adjustments	(1,278)
Foreign currency translation adjustments	23,751
Goodwill gross, ending balance	1,977,911	1,977,911
Accumulated impairment, ending balance	(40,866)	(40,866)
Goodwill, ending balance	1,937,045	1,937,045
Electric Power Infrastructure Services Division
Goodwill [Roll Forward]
Goodwill gross, beginning balance	1,253,979
Accumulated impairment, beginning balance	0
Goodwill, beginning balance	1,253,979
Goodwill acquired	5,866
Purchase price allocation adjustments	(619)
Foreign currency translation adjustments	14,767
Goodwill gross, ending balance	1,273,993	1,273,993
Accumulated impairment, ending balance	0	0
Goodwill, ending balance	1,273,993	1,273,993
Oil and Gas Infrastructure Services Division
Goodwill [Roll Forward]
Goodwill gross, beginning balance	388,923
Accumulated impairment, beginning balance	(39,733)
Goodwill, beginning balance	349,190
Goodwill acquired	305,537
Purchase price allocation adjustments	(659)
Foreign currency translation adjustments	8,984
Goodwill gross, ending balance	703,918	703,918
Accumulated impairment, ending balance	(40,866)	(40,866)
Goodwill, ending balance	$ 663,052	$ 663,052

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=77989000&loc=SL49117168-202975

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Amount of foreign currency translation gain (loss) which increases (decreases) an asset representing future economic benefits from other assets acquired in a business combination that are not individually identified and separately recognized.

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Amount of accumulated impairment loss for an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Amount of increase (decrease) from adjustments after acquisition date under purchase accounting of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
-Paragraph 16
-URI http://asc.fasb.org/extlink&oid=84229725&loc=d3e961-128460

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
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-Section 50
-Paragraph 1
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

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